1. Field of the Invention
The present invention relates to a polymer electrolyte of high durability and a production process thereof, more particularly to a polymer electrolyte of high durability which is suitable for an electrolyte membrane, an electrode material and the like for use in polymer electrolyte fuel cells, water electrolyzers, halogen hydracid electrolyzers, sodium chloride electrolyzers, oxide and/or hydrogen concentrators, humidity sensors, gas sensors and the like, and a production process thereof.
2. Description of Related Art
A polymer electrolyte is a polymer material having an electrolyte group such as a sulfonic acid group in a polymer chain. As the polymer electrolyte has properties of strongly bonding to specific ions and selectively permeating cations or anions, it is formed into particles, fibers or membranes, and utilized in various applications such as electrodialysis, diffusion dialysis and cell diaphragms.
For example, in the various electrochemical devices such as the polymer electrolyte fuel cells and the water electrolyzers, the polymer electrolyte is formed into a membrane and used in a state of a membrane-electrode assembly (MEA), where both sides of the polymer electrolyte are bonded with electrodes respectively. Also, in the polymer electrolyte fuel cell, the electrode generally has a two-layered structure consisting of a diffusion layer and a catalyst layer. The diffusion layer is for supplying the catalyst layer with a reaction gas and electrons, and carbon fibers, carbon papers or the like are used as the diffusion layer. Further, the catalyst layer acts as a reaction field of an electrode reaction, and generally consists of a complex of a carbon supporting a catalyst such as platinum and a polymer electrolyte.
As the polymer electrolyte for use in such applications, various materials are conventionally known. For example, for the electrolyte membrane and the electrolyte in the catalyst layer used in the electrochemical devices operating under a severe condition, a wholly fluorinated electrolyte membrane of excellent oxidation resistance is generally used (for example, “Nafion”, a registered trademark, manufactured by E. I. du Pont de Nemours & Company Inc., “Aciplex”, a registered trademark, manufactured by Asahi Kasei Corporation, “Flemion”, a registered trademark, manufactured by Asahi Glass Co., Ltd., and the like), and a hydrocarbon-based electrolyte is also considered for use.
Further, U.S. Pat. No. 3,624,053 discloses a trifluorovinyl sulfonic acid polymer which is obtained by copolymerizing trifluorovinyl sulfonyl fluoride (CF2═CFSO2F) and tetrafluoroethylene (CF2═CF2), a molecule of which includes a structure represented by —CF2—CF(SO3X)—(where X is hydrogen, alkaline metal, ammonium ions or amine ions), and equivalent weight of which is about 14000 g/eq (see column 1, lines 60-71 and column 2, lines 34-37).
Furthermore, U.S. Pat. No. 3,041,317 discloses a process of synthesizing fluoroalkenyl sulfonyl fluoride (RfCF═CFSO2F, where Rf is fluorine, perfluoroalkyl or omega-hydroperfluoroalkyl) which is used for synthesizing the trifluorovinyl sulfonic acid polymer described above (see column 1, lines 41-51).
Still further, Japanese Patent Application Unexamined Publication No. Hei4(1992)-11608 discloses a proton conductor represented by (—CF2—CF(SO3H)—)n (where n is 100 or more). Moreover, Japanese Patent Application Unexamined Publication No. 2002-105216 discloses sulfonated polychlorotrifluoroethylene obtained by sulfonation of a chloro group in polychlorotrifluoroethylene.
In an operating environment of the polymer electrolyte fuel cell, a peroxide is produced as a side reaction of an electrode reaction. Further, the produced peroxide becomes a peroxide radical while diffusing in the membrane. There has been a problem that  the conventional hydrocarbon-based electrolyte is prone to corrosion by the peroxide radical and has low oxidation resistance. This is because a hydrocarbon skeleton constituting the hydrocarbon-based electrolyte is easily subjected to an oxidative reaction by the peroxide radical. Therefore, the wholly fluorinated electrolyte of high chemical stability is generally used as the electrolyte for use in the fuel cell even it is very expensive.
However, inventors of the present invention have found that the wholly fluorinated electrolyte such as Nafion (the registered trademark) also deteriorates with time if it is used for a long time under the operating condition of the fuel cell. Also, a detailed analysis of a cause for the deterioration revealed that an ether linkage in the conventional wholly fluorinated electrolyte often becomes a source of the degradation.
Besides, the trifluorovinyl sulfonic acid polymer disclosed in U.S. Pat. No. 3,624,053 and U.S. Pat. No. 3,041,317 is known as an electrolyte having a structure not including the ether linkage in the molecule. However, there are problems that its equivalent weight is as high as about 14000 g/eq, and sufficient proton conductivity for operating the fuel cell cannot be secured. This is mainly due to low copolymerization reactivity of trifluorovinyl sulfonyl fluoride (CF2═CFSO2F), which is a monomer.
Further, Japanese Patent Application Unexamined Publication No. Hei4(1992)-11608 discloses the proton conductor represented by (—CF2—CF(SO3H)—)n (where n is 100 or more) as the electrolyte not including the ether linkage in the molecule. However, specific means for synthesizing such proton conductor is not disclosed therein at all.